Commercial compact discs (CDs) typically are recorded by impressing microscopic rectangular bumps in a clear polycarbonate plastic disc. The bumps are arranged as a single, continuous long spiral track of data circling from the center of the disk to the outside edge. The bumps are about 0.5 micrometers wide, at least 0.83 microns long and 125 nanometers high. A thin reflective coat of aluminum is sputtered onto the bumps, and an acrylic protective layer then is sprayed over the aluminum. A label is printed onto the acrylic layer.
Whereas commercially recorded CDs are stamped with data, CD-recordable (CD-R) discs can be “burned”, that is, written upon by a laser. A CD-R typically has a reflective surface coated by a translucent, optically active dye. During recording, the laser causes the dye to react and form opaque spots on the reflective surface.
Like CDs, digital versatile disks (DVDs) hold data that is encoded as lands and bumps along a track. DVDs typically are of the same size as CDs, but can hold two layers of data along a track. An inner data layer typically is sputtered with reflective aluminum, and an outer layer can be sputtered with semi-reflective gold. The inner layer can be read using a laser that penetrates the gold layer. A DVD can hold recorded data on each of its sides and thus can hold about seven times more data than can a CD.
When a CD, CD-R or DVD is played on a disc player, a laser beam follows a data track on the disc as the disc spins. The laser beam passes through a clear plastic layer of the disc and strikes a reflective layer. In the case of a CD or DVD, the laser strikes reflective bumps and low areas (lands) between the bumps. When a CD-R disc is played, the laser strikes reflective and opaque spots along the track. An opto-electronic sensor detects reflections of the laser. The reflections vary between lands and bumps (or, in the case of a CD-R disc, between reflective and opaque spots), and are interpreted by a processor as “1” or “0”.
It has become common to store data-intensive multimedia content on disc, thus making it desirable to provide as much storage capacity as possible on such devices. DVD technology provides a significant increase in data storage capacity over CDs and CD-Rs. Like CDs and CD-Rs, however, DVDs are limited by the fact that data bits are recorded in a linear fashion along a track. Current disc formats also reserve a significant number of data bits (for example, every eighth bit) for use by a tracking system as “locate” bits, thus precluding the bits from being used for data storage.